The invention relates to methods of fabricating highly uniform, ultra-small metallic micro-spheres or balls, and to the balls themselves, from capillary stream break-up at high rates.
The generation of droplets from capillary stream break-up has been studied at least as early as Lord Rayleigh in the 1800s. More recently, the formation of metallic micro-spheres, or balls, from the break-up of a molten metal capillary stream has been studied. Such balls are commonly used in the electronics industry for various applications, including interconnects for small electronics packages and in the manufacture of conductive pastes. Using the process of capillary stream break-up, the balls can be produced at very high rates xe2x80x94typically tens of thousands of droplets per second. Further, the nature of droplet formation due to capillary stream break-up results in highly uniform balls. The highly uniform size of the metal balls formed from capillary stream break-up is a significant improvement over other methods of forming conductive powdersxe2x80x94such as spray atomization or melt spinningxe2x80x94which require the extra step of sieving or sorting the differently sized balls. This extra step is labor intensive, significantly increasing the time and cost of the manufacturing process; however, with such technologies, sorting or sieving is necessary to achieve tight ball diameter tolerances (on the order of five percent).
In the production of metal balls from capillary stream break-up, it is advantageous to effectively cool the balls so that they solidify before landing or bonding with each other. Effective solidification reduces or eliminates (1) irregularly shaped balls that have dented when they impinge and (2) irregularly sized balls that have bonded together because they were insufficiently cooled. Without effective solidification, removal of these defects requires that the balls be sieved or sorted.
Conventional methods of formation of metal balls due to capillary stream break-up tend to be limited to metal balls having diameters in excess of 50 microns. A significant limitation on the size of metal balls produced from capillary stream break-up is the size of the orifice from which the capillary stream emerges. Typically, droplets generated from capillary stream break-up have diameters that are roughly twice as large as the diameter of the capillary stream orifice. The production of smaller balls, therefore, typically requires smaller orifices. As the orifice becomes very small, it tends to be more easily clogged by, e.g., impurities in the molten metal. Further, obtaining smaller orifices that are also uniform tends to be difficult and expensive. Current state-of-the-art provides a lower limit of orifice diameter available off-the-shelf and suitable for use with molten metals of 25 microns.
Accordingly, the present invention enables the formation of metallic micro-spheres due to capillary stream break-up that are significantly smaller than metallic micro-spheres formed by conventional methods and, more particularly, to metallic micro-spheres that are significantly smaller than the capillary stream orifice from which they emerge, thereby overcoming many of the difficulties that plagued the prior art by advantageously enabling the formation of much smaller micro-spheres from larger orifices. The present invention further enables forming highly uniform metalic micro-spheres or balls, having diameters on the order of about 1 to 100 microns, and preferably less than 25 microns, without the defects and difficulties associated with conventional methods.
A method of manufacturing ultra-small metallic spheres comprises directing a capillary stream of molten metal from an orifice by applying an excitation disturbance, wherein the excitation disturbance is determined so that parent droplets and satellite droplets form from the stream due to capillary stream break-up. In one innovative aspect of the present invention, the satellite droplets are separated from the parent droplets; cooled to form solid balls of substantially spherical shape; and collected as separate solid satellite balls. In another innovative aspect of the present invention, the satellite and parent drops are simultaneously cooled and collected as solid balls.
In one embodiment, the separating step is accomplished by electrostatically charging the droplets and directing them through an electric field, whereby the satellite and parent droplets deflect differently due to the different charge-to-mass ratios. In another embodiment, the droplets may be directed through a second electric field, a rotating field, or both to further disperse the droplets. In either of these embodiments, the electrostatic charge may vary over time while the electric field remains constant or the electric field may vary over time while the electrostatic charge remains constant.
In accordance with another embodiment, separation of the satellite and parent droplets is accomplished by acoustic forcing. In accordance with yet another embodiment, the satellite and parent droplets are separated with aerodynamic forces.
In another innovative aspect, a solid metal ball of the present invention has a diameter that is preferably substantially less than the diameter of the capillary orifice. In a further innovative aspect, a solid metal ball of the present invention is substantially spherical and has a diameter in a range of about 1.0 to 100 microns, and preferably less than 25 microns. In yet a further innovative aspect, a metallic powder comprises a plurality of such balls, wherein the balls are highly uniform having a ball diameter tolerance of a mean ball diameter in the range of about 0.5 to 3.0 percent, and preferably less than 2.0 percent, without performing a mechanical sieving or sorting step.
In another innovative aspect of the present invention, the metal balls, satellite or both satellite and parent, are produced at a rapid rate, wherein the balls are highly uniform, having highly uniform diameters. More particularly, the balls may be produced at a rate preferably in a range of about 1000 to 200,000 balls per second, and preferably at a rate greater than about 4000 balls per second while maintaining a ball diameter tolerance in the range of about 0.5 to 3.0 percent, and preferably a ball diameter tolerance of less than about 2.0 percent, without performing a mechanical sieving or sorting step.